[It's] a set of classes in the SPL that implements various iterating patterns: ArrayIterator, AppendIterator, FilterIterator, LimitIterator and NoRewindIterator. Hopefully you'll get a idea of what these are capable of and that you can get some new ideas for your day-to-day tasks.

He breaks it down into example of each, explaining what they can be used for, how they work and a code example of each in action (with output). You can find more information in iterators and their functions in the SPL section of the manual.